This invention relates to transformation of water-soluble metal sulfate residue obtained from spent sulfuric acid containing inorganic impurities into substantially water-insoluble solid form suitable for disposal.
In several industrial processes there are formed large quantities of by-product spent sulfuric acids containing dissolved metal sulfates. These acids, for example, include those obtained from hydrometallurgical leaching operations, from metal pickling operations such as steel pickling, from the process of making titanium dioxide pigment from titaniferous materials such as ilmenite and ilmenite slag, and the like. Such spent acids pose serious disposal problems.
It is already known to recover sulfuric acid from spent sulfuric acid containing metal sulfates. For example, F. J. Bartholomew describes a method for recovering sulfuric acid from spent sulfuric acid generated in steel pickling and titanium pigment production involving concentrating the acid by evaporization of water therefrom under crystallization of sulfate salts, and separation of these salts to recover acid for reuse. Bartholomew then goes further and recovers sulfur values from these precipitated salts by mixing them with coal, followed by heating to effect reduction of the sulfate to sulfur dioxide, under concomitant formation of metal oxides. [Ind. Eng. Chem. 44, p. 541 et seq. (1952)].
It is also known that various sulfates can be reacted with sulfur in dry state at suitable temperature to form sulfides and/or other reduction products [K. Bruckner, Ber. Wien. Adad. 114 IIa (1905) 1151/66; Monatsh. 27(1906) 40/58, 199/204].
Numerous processes for recovering sulfuric acid from metal sulfate-containing spent acids have been proposed involving evaporization of water from or distillation of the sulfuric acid component of such spent acids (see, e.g., U.S. Pat. No. 3,713,786). These evaporative processes, however, generally leave a residue of water-soluble metal sulfate, for which there are no satisfactory means of disposal.
It is an object of the present invention to provide a process for tranformation of water-soluble metal sulfate residue obtained from spent sulfuric acid containing inorganic impurities into a substantially water insoluble residue suitable for disposal without threat to the environment.